Title: Granulometric and lithologic control on heavy mineral distribution in Alpine fluvial sediment

Ariane Djahansouzi1, Laura Stutenbecker1, Daniela Krieg1, Christoph Glotzbach2

1Technical University of Darmstadt, Darmstadt, Germany; 2Eberhard Karls University of Tübingen, Tübingen, Germany

Event: ECSM 2021

Date: 2021

Heavy mineral studies are dependent on many variables that should be considered in order to avoid erroneous provenance interpretations. Firstly, sediment transported and deposited by larger rivers is the result of the erosion of several lithological units present in the catchment with potentially differing mineral fertilities. Secondly, possible bias arises from the narrow sand-size window that is typically considered for single-grain analysis in sedimentary provenance studies. Quantifying and understanding the resulting bias is crucial for the correct interpretation, especially of single-grain methods such as apatite or zircon geochronology.

In this study, an inter- and intra-sample comparison of apatite and zircon concentrations is conducted on modern Alpine fluvial sands derived from five monolithological catchments draining granitoid, ophiolitic, metamorphic and sedimentary sources. The distribution of these minerals was quantified and compared within narrow grain size windows of each sample using different methods: (1) Point counting in strewn slides, (2) QEMSCAN analysis, (3) concentration of apatite and zircon using magnetic as well as dense liquid separation, (4) XRF and ICP-MS analysis of P2O5 and Zr as proxies for apatite and zircon, respectively, and (5) modelling based on the size shift.

While in line with published fertility values in the Alps, the inter-sample results show varying heavy mineral concentrations over three orders of magnitude in different source lithologies. They tend to be highest in metamorphic and lowest in sedimentary source lithologies. The intra-sample comparison shows highest zircon and apatite concentrations in the finer sand size fraction from 63-250 μm, which is expected from the settling-equivalence principle. However, the geochemical analysis also reveals a significant amount of P2O5 and Zr within the grain sizes smaller than 63 μm. This is especially important, since many single-grain provenance studies do not consider the silt and clay fractions and thus miss those apatite and zircon populations.

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